Radiant floor heat under 3x8 feet wide strip of tile - how to build up rest of floor?

I just realized title isn't correct - the tile will cover the bathroom floor but I only want a 3x8 foot wide strip of radiant heating.

My bathroom floor is an L shape (tub is the reason for the L). If I do electric radiant floor heat, I figure I need the pad only down a 3 feet wide x 8 foot long strip (doesn't need to go behind toilet, under vanity, or behind door).

The pad is supposed to be 1/8" high, but with thinset I'm guessing it gets close to a 1/4 inch? How do you build up the rest of the floor to match? (I've been planning granite for the tile, but may switch to ceramic, given the bigger headaches with granite.)

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Radiant floor heat under 3x8 feet wide strip of tile - how to build up rest of floor?

Self Leveling Compound will definitely be easiest. You need to use one designed for your subfloor. If it's concrete, any will do. If it's wood you need to use one rated for the type of wood. TEC EZ Level for example is one of the only ones I've seen that you can use on OSB. Regardless, to do it completely by the book:
1) Use a minimal expanding spray foam to run and expansion bead around the walls and to seal any areas the SLC may run when poured.
2) Prime the subfloor with the SLC primer
3) Install the heat.
4) Install lath in the areas that don't get the pad. I use Mapelath, a plastic mesh that helps strengthen the structure of the SLC.
5) Mix and pour the SLC. Trowel it out with a float and let it level the rest of the way on it's own.
6) Install crack isolation membrane, something like Ditra.
7) Lay tile and grout.